The exemplary embodiment relates to transport devices for print media and finds particular application in connection with a transport member for an inkjet printing system which generates an electrostatic field or other electric field for transporting print media.
Inkjet printing systems generally include a printhead which applies a liquid ink composition from an array of inkjets to form an image on a sheet print media, such as paper. Following deposition of the ink, the image is dried or cured. Aqueous inks are often used which include a significant proportion of water and typically 5-15 weight % of co-solvents with boiling points above 200° C. The water is removed by drying the sheet with airflow and heat or infrared radiation, however, the co-solvents are best left to penetrate into the paper. When coated papers are used with aqueous ink, the co-solvent penetration rates are lower, due to reduced surface porosity. Glossy coated papers can be as low as 3% surface porosity.
Sheets of print media are conveyed through the printer by a sheet transport system, which may include a combination of transport members, such as nip rollers, transport belts, and the like. Problems can occur in the transport system, during drying, depending on the type of transport member used. In the case of nip rollers, these can damage the wet image through contact. Failure to fully dry the image before touching the rollers also causes roll contamination, requiring manual cleaning. Currently, aqueous inkjet systems are designed to dry the image to touch before engaging any nipped drive rollers. In the case of vacuum transport belts, which apply suction to the sheet from below, uneven heating of the sheet can occur. The effects of variation in heating rate and final achieved temperature are particularly noticeable in the image on coated media, which may be evident as a density shift or a gloss shift. Further, vacuum transports have a limited latitude to acquire and hold the sheet leading to the need to reduce the air flows in the dryer oven, especially while the first or last sheet enter or exit the dryer, when most of the transport vacuum holes are uncovered.
There remains a need for a sheet transport system which facilitates drying of the sheets while minimizing these problems, and others.